The cessation of protoplasmic streaming during mitosis in the plasmodium of the myxomycete Physarum polycephalum will be examined as a model system for studying the regulation of actomyosin activity in non-muscle systems. To test the idea that a decrease in free calcium ion during mitosis is responsible for the cessation of streaming, plasmodia will be incubated with the calcium ionophore A23187 to see if streaming continues during mitosis. The state of polymerization of the mitotic spindle will also be examined by electron microscopy. To test the idea that myosin light chain phosphorylation is part of the regulatory process, partly purified preparations of Physarum actomyosin will be incubated in the appropriate conditions with gamma-32P labeled ATP. The proteins will be separated into bands by SDS-polyacrylamide gel electrophoresis, followed by autoradiography, to determine if myosin light chain becomes phosphorylated.